Method To Control The Effects Of Out-Of-Cell Interference In A Wireless Cellular System Using Over-The-Air Feedback Control

ABSTRACT

A mobile terminal is controlled via over-the-air feedback so as to enable its data transmissions to be independently and successfully decoded at each of the base stations in its active set absent a transmit power limitation or data retransmission limit. Using the decoded data, the channel is re-estimated and the waveform received from the mobile terminal is reconstructed and subtracted from the total interference at each base station in the active set where decoding has been successful. As a result, transmissions from other mobile terminals, which have yet to be successfully decoded at such a base station, will experience a higher signal-to-noise ratio and thus an increased likelihood of being successfully decoded.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation of U.S. application Ser. No.11/409,161 filed on Apr. 21, 2006, to Shirish Nagaraj, et al., nowissued as U.S. Pat. No. 8,493,941, commonly assigned with the presentinvention and incorporated herein by reference in its entirety.

TECHNICAL FIELD

This invention relates to wireless communications.

BACKGROUND OF THE INVENTION

In a wireless system, mobile terminals transmit and receive data overbi-directional wireless links from one or more base stations. The mobileterminal-transmit direction is known as the uplink and the mobileterminal-receive direction is known as the downlink. The set of basestations with which a mobile terminal is communicating is known as theactive set of base stations for that mobile terminal, or that mobileterminal's active set. During normal conditions when a mobile terminalis within a base station's coverage area, the active set for that mobileterminal would generally be a single base station so that the active setincludes only that one base station, which is that mobile terminal'sserving station. When a mobile terminal, however, is within the range ofmultiple base stations and as such is in a handoff state, the active setincludes the multiple base stations, which are each monitoring thesignal from that mobile terminal and decoding it when able to do so.Only one of the base stations, however, is the serving base station forthat mobile terminal and the other base stations are non-serving basestations. FIG. 1 shows three base stations 101, 102 and 103 and theirrespective cell coverage areas 104, 105 and 106. Mobile terminal 107 isshown within the coverage area 104 of base station 101, which is itsserving base station. Non-serving base stations 102 and 103, however, inaddition to serving base station 101 constitute the active set formobile terminal 107.

In a data system, on the downlink, the mobile terminal 107 receives datafrom only one base station but has the option of reselecting the servingbase station in order to receive data from any other base station in itsactive set depending on from which base station the mobile terminalreceives a signal with the highest signal-to-noise ratio. On the uplink,the serving and the non-serving base stations each attempts todemodulate and decode transmissions from the mobile terminal.

The capacity of a wireless system refers either to the number of mobileterminals that can simultaneously transmit or receive data, or theaggregate date rate of these mobile terminals, either expressed inmobile terminals/sector, erlangs/sector or data throughput/sector. Theuplink capacity of the system can be different from its downlinkcapacity. For symmetric services, such as voice, (i.e., requiredthroughput/data rate for a mobile terminal on the uplink is equal tothat on the downlink), the overall system capacity is limited by thelower of uplink and downlink capacity. In current wireless systemsspecified by standards such as CDMA2000 1 x, EV-DO Rev 0 and Rev A,HSDPA/EDCH, and WiMAX, the uplink has a substantially lower capacitythan the downlink. This imbalance needs to be remedied for full use ofdownlink capacity and to maximize the number of mobile terminals thancan operate symmetric services on the system.

In wireless systems that are based on direct spread or multi-carrier(optionally with precoding) CDMA, a plurality of mobile terminals withina sector (and across sectors) re-use a spreading sequence or a set offrequency tones to communicate with their respective active sets, whilebeing differentiated by mobile terminal-specific codes. A mechanism forincreasing uplink sector capacity is to perform successive interferencecancellation on these transmissions at the base station transceiver.FIG. 2 illustrates base station noise rise components. As shown, at abase station receiver 200, the total rise over thermal noise in a sectorconsists of the composite signal 201 from the mobile terminals withinthat sector for which that base station is the serving base station, andthe out-of-cell interference 202 caused by mobile terminals transmittingin adjacent sectors. The latter includes interference from those mobileterminals in the adjacent sectors for which base station receiver 200 iswithin these mobile terminals' active set but for which base station 200is non-serving, plus the interference caused by other transmittingmobile terminals in other sectors that do not include base station 200within each such mobile terminal's active set.

An illustrative method of interference cancellation is disclosed in U.S.Pat. No. 7,385,944. Using such an interference cancellation method, ifthe decoding of any mobile terminal is successful, its signal isreconstructed and subtracted from the composite received signal at thebase station. FIG. 3 shows a successive interference cancellation schemeat an exemplary base station receiver 300 that is the serving basestation for four mobile terminals 301, 302, 303 and 304 within a sectorof that base station receiver 300. The received power at base stationreceiver 300 from mobile terminals 301, 302, 303, and 304 isrespectively P_(—)1, P_(—)2, P_(—)3, and P_(—)4. In addition, basestation receiver 300 receives a composite signal power as the result ofout-of-cell interference (IOC) caused by transmissions from mobileterminals out of the sector. When a particular transmission from amobile terminal from within the sector is successfully decoded by thebase station receiver, the transmission is reconstructed and subtractedfrom the composite signal at the base station, after which anotherreceived signal is demodulated, decoded, reconstructed and subtractedfrom the remaining composite signal. This process is repeated for eachof the remaining signals. Advantageously, the signals from the mobileterminals that are decoded later in the demodulation and decodingprocess do not “see” the interference from transmissions from the mobileterminals that were decoded earlier in the sequence. FIG. 3 shows asuccessive calculation of the signal-to-noise ratios (Snr_(—)1-Snr_(—)4)of the four mobile terminals 301-304, respectively. Starting with mobileterminal 301, Snr_(—)1 is calculated asP_(—)1/(P_(—)2+P_(—)3+P_(—)4+IOC+N), where N is the measurable thermalnoise. The contribution from each is successively subtracted off fromthe received composite signal at the base station receiver, so that, forthe last mobile terminal 304, Snr_(—)4 is calculated as P_(—)4/(IOC+N).Since the mobile terminals that are decoded later see a highersignal-to-noise ratio, they are capable of supporting either a higherrate of transmission and/or increased reliability.

In the above-described scenario, it is not possible for the base stationreceiver to successfully decode the transmissions of all the mobileterminals that have this base station sector in their active set. As aresult, most of the out-of-cell interference received by a base stationreceiver cannot be deducted. Thus, as noted above, the signal-to-noiseratio for station 304 is still limited by this out-of-cell interferenceIOC.

Typically, the power control rule followed by mobile terminals is toeither (i) follow power control commands from the serving sector in itsactive set, or (ii) follow a rule known as the or-of-the downs, wherebythe mobile terminal lowers its power if any of the base stations in theactive set instructs it to do so via a down power control command. Whilean or-of-the-downs power control ensures successful reception of themobile terminal's transmission at [[at]] least one base station(presumably the one with the best uplink connection from the mobileterminal), it also ensures that the mobile terminal's transmission isnot received with adequate signal-to-noise ratio to be successfullydecodable at all of the base stations in the active set. Thus, thisundecodable interference limits the capacity gain from a systememploying successive interference cancellation. Even as mobile terminalswithin a sector transmit with ever increasing powers, in order toincrease their signal-to-noise ratios at the base station receiver (andhence achieve higher data rates), their interference to adjacent sectorsgrows proportionately, thereby limiting the rates that can be achievedby mobile terminals in those sectors. In turn, the interference from themobile terminals in adjacent sectors marginalizes the gains for themobile terminals with the sector under consideration that increasedtheir power in the first place.

A methodology is thus desired that enables a base station receiver toreconstruct and subtract the out-of-cell interference from the compositereceived signal so that the signal-to-noise ratio can be improved forall in sector mobile terminals.

SUMMARY OF THE INVENTION

In an embodiment, a device, e.g. a mobile terminal, includes a receiverand a transmitter. The receiver is configured to receive commands from aplurality of base stations. The transmitter is configured to send, whenthe a mobile terminal is in handoff status, data packets to theplurality of base stations in the mobile terminal's active set of basestations, wherein the active set including a serving base station and atleast one non-serving base station. The transmitter is furtherconfigured to increase the transmit power of the transmitter, inresponse to the receiver receiving a command to increase the transmitpower from any of the base stations, unless the transmit power is at itsmaximum power or the transmit power exceeds a power targeted by theserving base station by a predetermined amount.

Another embodiment is a method, e.g. for interference management in awireless communication system. The method includes sending, at a mobileterminal in handoff status, data packets to a plurality of base stationsin the mobile terminal's active set of base stations, wherein the activeset includes a serving base station and at least one non-serving basestation. The method further includes increasing the mobile terminal'stransmit power, in response to receiving a command to increase thetransmit power from any of the base stations in the active set, unlessthe transmit power is at its maximum power or the transmit power exceedsa power targeted by the serving base station by a predetermined amount.

In any embodiment of the aforementioned device or method, the transmitpower may be increased by a predetermined up-step. In any embodiment,the transmitter may be configured to increase or decrease the transmitpower only in response to commands received from the serving basestation, on the condition that: the transmit power is at its maximumpower; the mobile terminal receives a command to increase its transmitpower from any of the base stations in the active set, and the transmitpower exceeds a power targeted by the serving base station by apredetermined amount.

In any embodiment the transmitter may be configured to increase thetransmit power only if the receiver determines the received command isdetermined to be reliable. In such embodiments the receiver maydetermine that the command to increase the transmit power is reliable onthe condition that a power of the received command to increase the poweris greater than a predetermined threshold.

In another embodiment a device, e.g. a mobile terminal, includes areceiver and a transmitter. The receiver is configured to receivecommands from a plurality of base stations. The transmitter isconfigured to send, when the mobile terminal is in handoff status, datapackets to a plurality of base stations in an active set of basestations, the active set including a serving base station and at leastone non-serving base station. The receiver is further configured toincrease a transmit power in response to the receiver receiving anegative acknowledgment (NACK) of a successful decoding of a givenpacket received from any of the base stations at the end of thatpacket's retransmission limit, unless the transmit power is at itsmaximum power or the transmit power exceeds a power targeted by theserving base station by a predetermined amount.

Yet another embodiment is a method, e.g. for interference management ina wireless communication system. The method includes sending, at amobile terminal in handoff status, data packets to a plurality of basestations in its active set of base stations, wherein the active setincludes a serving base station and at least one non-serving basestation. The method further includes increasing the mobile terminal'stransmit power, in response to receiving a negative acknowledgment(NACK) of a successful decoding of the packet from any of the basestations in the active set at the end of a given packet's retransmissionlimit, unless the transmit power is at its maximum power or the transmitpower exceeds a power targeted by the serving base station by apredetermined amount.

In any embodiment of the device or method described immediately above,the transmitter may be configured to increase the transmit power by apredetermined up-step. In any such embodiment, if the transmit power isat its maximum power or if the transmit power exceeds a power targetedby the serving base station by a predetermined amount, and the receiverreceives from any of the base stations in the active set a command toincrease the transmit power, then for a subsequent data transmission thetransmitter is configured to increase the transmit power only if thereceiver receives a NACK from the serving base station and to decreasethe transmit power if the receiver receives an ACK from the serving basestation. In any embodiment of the device or method described immediatelyabove, the transmitter may be configured to increase the transmit poweronly if the receiver determines a NACK received from any of the basestations in the active set is reliable. In any embodiment of the deviceor method described immediately above the receiver may determine theNACK is reliable by comparing a power of the received NACK with apredetermined threshold.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will be better understood from reading thefollowing description of non-limiting embodiments, with reference to theattached drawings, wherein below:

FIG. 1 shows a prior art arrangement of a mobile terminal and theserving base station and non-serving base stations in its active set;

FIG. 2 illustrates base station noise rise components;

FIG. 3 shows a prior art successive interference cancellation scheme atan exemplary base station receiver that is the serving base station forfour mobile terminals within a sector of that base station receiver;

FIG. 4 shows the steps at a mobile terminal in accordance with a firstembodiment that employs an or-of-the ups rule in controlling itstransmit power during handoff;

FIG. 5 shows the steps at a mobile terminal in accordance with a secondembodiment that employs an or-of-the-NACKS rule in controlling itstransmit power during handoff; and

FIG. 6 shows the steps at a mobile terminal in accordance with a thirdembodiment that employs an or-of-the-NACKS rule in controlling itsretransmissions of data during handoff.

DETAILED DESCRIPTION

With reference to the flowchart in FIG. 4, the methodology employed at amobile terminal operating in accordance with an or-of-the-ups rule isshown. At step 401, a determination is made whether the mobile terminalis in handoff (i.e., whether or not the active set is greater than one).In addition, but not shown, an optional additional determination may bemade of whether the mobile terminal's average path losses to the basestations in its active set differ from each other by more than apredetermined amount. If it is not in handoff, at step 402, the mobileterminal follows power control commands received from its one servingbase station. If it is in handoff (and if the optional determination ofaverage path losses indicates that such average path losses differ fromeach other by less than the predetermined amount), then, at step 403,the mobile terminal monitors power control commands from the basestations in its active set and accumulates power control commands (thesequence of +1 s and −1 s) from the serving base station to determinethe power targeted by the base station. Specifically, by accumulatingthese +1 s and −1 s, the power level at which the serving base stationwould like the mobile terminal to operate can be determined. At step404, a determination is made whether any of the power commands receivedby any of the base stations in the active set is an “up” (an or-of-theups rule). If not, indicating that the received power level at all ofthat base stations in the active set is at or above the target (and inturn that the transmission is likely to have been successful at all ofthe base stations), then, at step 405, the mobile terminal decreases itstransmit power by a predetermined down-step. If, at step 404, any of thepower commands received from any of the base stations in the active setis an “up” (indicating that transmission in unlikely to have beensuccessful at the base station(s) from which the up is received), thenthe mobile terminal transmit power may be increased. Before increasingits transmit power, however, a determination is made, at step 406,whether the mobile terminal is already at its maximum transmit power, orwhether the mobile terminal transmit power exceeds the power targeted bythe serving base station by more than a predetermined amount. If eitheris the case, then, at step 407, the mobile terminal reverts to anor-of-the-downs power control rule (i.e., the mobile terminal lowers itspower if any base station in the active set instructs it to do so).Alternatively, but not shown, the mobile terminal reverts to followingpower control from only the serving base station. If, at step 406, themobile terminal is not at its maximum transmit power and does not exceedthe power targeted by the serving base station by the predeterminedamount, then, at step 408, the mobile terminal increases its transmitpower by a predetermined up-step.

With reference to the flowchart in FIG. 5, the methodology employed at amobile terminal operating in accordance with an or-of-the-NACKS rule isshown. At step 501, a determination is made whether the mobile terminalis in handoff. As in the first embodiment, an optional additiondetermination may be made of whether the mobile terminal's average pathlosses to the base stations in its active set differ from each other bymore than a predetermined amount. If it is not in handoff, at step 502,the mobile terminal follows acknowledgment feedback (ACKS/NACKS) fromits one serving base station. If it is in handoff, (and if the optionaldetermination of average path losses indicates that such average pathlosses differ from each other by less than the predetermined amount),then, at step 503, at the end of the packet transmission limit (i.e.,after the mobile terminal has transmitted a data packet its maximumallowed number of times), the mobile terminal monitors acknowledgmentsfrom all base stations in the active set. In addition, the transmitpower of the mobile terminal that is required for a successful receptionat the serving base station is tracked. At step 504, a determination ismade whether at the end of the packet transmission limit any of theacknowledgments is a NACK, indicating an unsuccessful reception (anor-of-the-NACKS determination) at the base station(s) from which theNACK(s) is received. It should be noted that in certain systems a NACKis indicated by base station silence, i.e., the absence of the basestation transmitting an ACK. If none of the responses is a NACK, thenthe packet has been successfully received at each of the base stationsin the active set and, at step 505, the mobile terminal power isdecreased by a down-step for a next packet transmission. If, however, atstep 504, any of the base station responses is a NACK, then the mobileterminal power may be increased. Before increasing its transmit power,however, a determination is made, at step 506, whether the mobileterminal is already at its maximum transmit power, or whether the mobileterminal transmit power exceeds the power targeted by the serving baseby more than a predetermined amount. If either is the case, then, atstep 507, the mobile terminal reverts to an or-of-the-ACKS power controlrule for subsequent packet transmissions (i.e., the mobile terminalincreases its transmit power only if doesn't receive an ACK from atleast one base station in its active set). Alternatively, but not shown,the mobile terminal reverts to following power control based on theACK/NACK received from only the serving base station. If, at step 506,the mobile terminal is not at its maximum transmit power and does notexceed the power targeted by the serving base station by thepredetermined amount, then, at step 508, the mobile terminal increasesits transmit power by a predetermined up-step.

With reference to the flowchart in FIG. 6, another methodology employedat a mobile terminal operating in accordance with an or-of-the-NACKSrule is shown. At step 601, a determination is made whether the mobileis in handoff. As in the previously discussed embodiments, in addition,but not shown, an additional optional determination may be made ofwhether the mobile terminal's average path losses to the base stationsin its active set differ from each other by more than a predeterminedamount. If it is not in handoff, at step 602, the mobile terminalfollows acknowledgment feedback from its one serving base station. If itis in handoff, (and if the optional determination of average path lossesindicates that such average path losses differ from each other by lessthan the predetermined amount), then, at step 603, the mobile terminalmonitors acknowledgments from all the base stations in its active set.At step 604, a determination is made whether any of the base stationresponses to a packet transmission is a NACK. If not, at step 605, themobile terminal prepares to transmit the next data packet. If, however,at step 604, it does receive a NACK from one or more base stations inits active set, the mobile terminal may retransmit the packet. If, atstep 606, the retransmission limit has not been reached for this packet,then, at step 607, the mobile terminal retransmits the data packet. If,however, at step 606 the retransmission limit has been reached for thispacket, then, at step 608, retransmission for subsequent packets isperformed based on an or-of-the-ACKS rule wherein the packet is notretransmitted if an ACK is received from any base station in the activeset. Alternatively, but not shown, if the retransmission limit has beenreached, retransmission for subsequent packets may be based on theACK/NACK acknowledgments received only from the serving base station.

In each of the embodiments described above, the mobile terminal mayperform a test to determine the reliability of the ACKS or NACKS, orpower control bits that it receives from the base stations in its activeset in order to determine whether or not to raise its transmit power orto retransmit a packet. Specifically, the received power of the receivedACKS/NACKS or power control bits can be compared against a threshold. Inthose systems in which ACKS and NACKS are both positively transmitted,then if the power is greater than the threshold, then the receivedACKS/NACKS or power control bits can be assumed to be reliable and usedto make the appropriate determination of increasing/decreasing thetransmit power or retransmitting the packet. On the other hand, if thepower is below the threshold, a decision to increase the power orretransmit the packet is made only on the basis of those ACKS/NACKS orpower control bits that are deemed to be reliable. For those systems inwhich a NACK is indicated by base station silence a NACK would be deemedto be reliable if its power was below a threshold.

Upon a successful decoding of a mobile station's data transmission thereceived signal from the mobile terminal is reconstructed and subtractedoff from the composite received signal prior to decoding other mobileterminal's transmissions with a concomitant improved signal-to-noiseratio for these other mobile terminals' transmissions. For mobileterminals that have adopted the fallback position for power control orACK/NACK feedback, the amount of interference presented to thenon-serving base stations is likely to be decreased and the penalty dueto not being able to successfully decode those transmissions is reduced.

As previously discussed, the described methodology can be used in anysystem where out-of-cell interference limits the signal-to-noise ratiofor one or more in-cell mobile terminals. Examples of such systeminclude a system employing successive interference cancellation or anOFDMA system that allows only one in-cell transmission over a given setof frequency tones.

The above-described embodiment is illustrative of the principles of thepresent invention. Those skilled in the art can devise other embodimentswithout departing from the spirit and scope of the present invention.

1-20. (canceled)
 21. A mobile terminal, comprising: a receiverconfigured to receive commands from a plurality of base stations; and atransmitter configured to: send, when the a mobile terminal is inhandoff status, data packets to the plurality of base stations in themobile terminal's active set of base stations, the active set includinga serving base station and at least one non-serving base station; andincrease the transmit power of the transmitter, in response to thereceiver receiving a command to increase the transmit power from any ofthe base stations, unless the transmit power is at its maximum power orthe transmit power exceeds a power targeted by the serving base stationby a predetermined amount.
 22. The mobile terminal of claim 21 whereinthe transmit power is increased by a predetermined up-step.
 23. Themobile terminal of claim 21 wherein: in the event that: the transmitpower is at its maximum power, or the transmit power exceeds a powertargeted by the serving base station by a predetermined amount, and themobile terminal receives a command to increase its transmit power fromany of the base stations in the active set, the transmitter isconfigured to increase or decrease the transmit power only in responseto commands received from the serving base station.
 24. The mobileterminal of claim 21 wherein the transmitter is configured to increasethe transmit power only if the receiver determines the received commandis determined to be reliable.
 25. The mobile terminal of claim 24wherein the receiver determines that the command to increase thetransmit power is reliable on the condition that a power of the receivedcommand to increase the power is greater than a predetermined threshold.26. A mobile terminal, comprising: a receiver configured to receivecommands from a plurality of base stations; and a transmitter configuredto: send, when the mobile terminal is in handoff status, data packets toa plurality of base stations in an active set of base stations, theactive set including a serving base station and at least one non-servingbase station; and increase a transmit power in response to the receiverreceiving a negative acknowledgment (NACK) of a successful decoding of agiven packet received from any of the base stations at the end of thatpacket's retransmission limit, unless the transmit power is at itsmaximum power or the transmit power exceeds a power targeted by theserving base station by a predetermined amount.
 27. The mobile terminalof claim 26 wherein the transmitter is configured to increase thetransmit power by a predetermined up-step.
 28. The mobile terminal ofclaim 26 wherein: if the transmit power is at its maximum power or ifthe transmit power exceeds a power targeted by the serving base stationby a predetermined amount, and the receiver receives from any of thebase stations in the active set a command to increase the transmitpower, then for a subsequent data transmission the transmitter isconfigured to increase the transmit power only if the receiver receivesa NACK from the serving base station and to decrease the transmit powerif the receiver receives an ACK from the serving base station.
 29. Themobile terminal of claim 26 wherein the transmitter is configured toincrease the transmit power only if the receiver determines a NACKreceived from any of the base stations in the active set is reliable.30. The mobile terminal of claim 26 wherein the receiver determines theNACK is reliable by comparing a power of the received NACK with apredetermined threshold.
 31. A method for interference management in awireless communication system, the method comprising: at a mobileterminal in handoff status, sending data packets to a plurality of basestations in the mobile terminal's active set of base stations, theactive set including a serving base station and at least one non-servingbase station; and in response to receiving a command to increase thetransmit power from any of the base stations in the active set,increasing the mobile terminal's transmit power, unless the transmitpower is at its maximum power or the transmit power exceeds a powertargeted by the serving base station by a predetermined amount, and 32.The method of claim 31 wherein the mobile terminal's transmit power isincreased by a predetermined up-step.
 33. The method of claim 31 whereinif the transmit power is at its maximum power or if the transmit powerexceeds a power targeted by the serving base station by a predeterminedamount, and the mobile terminal receives a command to increase itstransmit power from any of the base stations in the active set, thencontrolling the transmit power up or down only in response to commandsreceived from the serving base station.
 34. The method of claim 31further comprising determining the reliability of a received command toincrease the transmit power, and increasing the transmit power only ifthe received command is determined to be reliable.
 35. The method ofclaim 34 wherein the command to increase the transmit power isdetermined to be reliable if a power of the received command to increasethe power is greater than a predetermined threshold.
 36. A method forinterference management in a wireless communication system comprising:at a mobile terminal in handoff status, sending data packets to aplurality of base stations in its active set of base stations, theactive set including a serving base station and at least one non-servingbase station; and in response to receiving a negative acknowledgment(NACK) of a successful decoding of the packet from any of the basestations in the active set at the end of a given packet's retransmissionlimit, increasing the mobile terminal's transmit power unless thetransmit power is at its maximum power or the transmit power exceeds apower targeted by the serving base station by a predetermined amount.37. The method of claim 36 wherein the mobile terminal's transmit poweris increased by a predetermined up-step.
 38. The method of claim 36wherein: if the transmit power is at its maximum power or if thetransmit power exceeds a power targeted by the serving base station by apredetermined amount, and the mobile terminal receives a command toincrease its transmit power from any of the base stations in the activeset, then for a subsequent data transmission the mobile terminalincreases its transmit power only if it receives a NACK from the servingbase station and decreases its transmit power if it receives an ACK fromthe serving base station.
 39. The method of claim 36 further comprisingdetermining the reliability of a received NACK from any of the basestations in the active set, and increasing the transmit power only ifthe received NACK is determined to be reliable.
 40. The method of claim36 wherein the NACK is determined to be reliable by comparing a power ofthe received NACK with a predetermined threshold.